This invention relates to an abrasive water jet (xe2x80x9cAWJxe2x80x9d) peening surface treatment for providing a textured surface with specific surface topography on metals. In particular, this invention relates to the use of a high velocity AWJ as a mechanical surface treatment process that simultaneously textures and work hardens the surface of a metal substrate through controlled hydrodynamic erosion. One embodiment of this invention can be used to provide a textured surface on metal orthopedic implants.
Total joint arthroplasty is one of the most common surgical treatments for those who suffer from debilitating arthritis. Though successful at restoring joint mobility, clinical surveys have determined that the long-term success of total joint arthroplasty is often impaired by the loss of fixation between the prosthesis and the bone in cementless arthroplasty or between the prosthesis and the bone cement. Mechanical loosening of implants from the bone or cement can result in excessive joint displacement and generally mandates the need for revision surgery. Therefore, the development of stable primary fixation is a critical requirement for successful total joint arthroplasty.
Porous coatings can be applied (or added) to the surface of prosthetic devices to foster stable device fixation and is the conventional means of providing a textured surface for the bonding of implants to the bone. The coating serves as a source for mechanical interlocking and may stimulate healthy bone growth through Osseo integrated load transfer in cementless arthroplasty. Interestingly, some researchers have concluded that the surface texture of prosthetic devices may be designed to maximize the rate and extent of fixation through healthy cell growth. Three of the most common surface coatings used for metal implants include sintered beads, diffusion bonded wire mesh, and metallic plasma sprays. Clinical reports have clearly substantiated the benefits of these surfaces to the long-term success of implanted components. According to intermediate post-surgery follow ups, porous coated components have the ability to maintain fixation with probability of survival exceeding 0.95.
However, there is a substantial problem with the use of these coatings. Although porous coatings are considered a requirement for stable primary fixation, the fatigue strength of porous coated titanium and cobalt chrome devices (Ti6Al4V and CoCrMo) is sacrificed. The reduction in fatigue strength results from the stress concentration posed by the porous surface topography and through microstructural changes invoked during coating deposition. Together these features facilitate accelerated fatigue crack initiation in the surface of a device with inferior fatigue resistance. Early component failures, within 1 to 3 years post operative, are nearly always associated with fatigue crack initiation at the textured surface. Although increasing the component size can reduce stresses in vivo and extend the prosthesis fatigue life, stress shielding and bone resorption may develop due to the corresponding increase in component stiffness. Therefore, an alternative method of surface treatment is sought which supports stable fixation without sacrificing the component""s fatigue strength.
The long-term success of total joint arthroplasty requires the development of stable primary fixation. Consequently, the device surface texture and apparent volume available for bone ingrowth and/or cement interdigitation is of critical importance. Apart from the importance of fixation, the component fatigue strength must exceed that required to achieve an infinite life with acceptable reliability. Hence, the apparent stress concentration resulting from the component surface texture is an important factor and may be detrimental to the prosthesis fatigue strength.
Furthermore, according to principles of solid mechanics, surfaces with a high effective stress concentration will generally exhibit a relatively short fatigue life. Thus, it is advantageous to maximize the volume available for interdigitation through the implant surface topography while simultaneously minimizing the apparent stress concentration. In addition to the influence of stress concentrations, residual stresses are also important to the fatigue strength of orthopedic implants. Residual stresses within the prosthesis resulting from surface treatments may superpose with stresses imposed by external loads carried through the joint. A compressive residual stress serves to reduce the effective stress at the component surface and is generally found to increase the fatigue life of metals. Conversely, tensile residual stresses are detrimental. Plasma spray treatments of metal implants result in tensile residual stresses within the coating surface and therefore may reduce the component fatigue strength. Although post-process heat treatments can be used to relieve tensile residual stresses, it would be advantageous to generate a compressive residual stress within the textured surface of implants during primary processing.
It is an object of the present invention to provide a method and apparatus to achieve an abrasive water jet peening surface treatment that overcomes the problems encountered in the prior art discussed above.
Specifically, the present invention provides a method and apparatus that includes supporting a metal workpiece on a workpiece support and arranging a nozzle above a target surface of the workpiece supported on the workpiece support so that the nozzle is pointed towards the target surface of the workpiece. A pressurized fluid is then generated by a device such as a pump, and abrasive particles are entrained within the pressurized fluid. The pressurized fluid having the entrained abrasive particles is discharged through the nozzle and toward the target surface of the workpiece. The nozzle is located a texturing standoff distance from the target surface such that the periphery of the pressurized fluid stream discharged from the nozzle expands after being discharged from the nozzle and prior to impinging upon the target surface of the workpiece. As a result, a textured (i.e., deformed) and hardened surface is created on the workpiece, and the textured surface allows the workpiece to be securely fixed to bone in the form of a prosthesis, while the hardening significantly increases the effective life of the workpiece.
Several types of abrasive particles can be used with the method and apparatus of the present invention. In particular, biocompatible abrasive particles, such as hydroxyapatite, can be used for stimulation of bone growth. In addition, garnet abrasive particles can also be used, and the particles can have various sizes depending on the desired results. The abrasive particles can be supplied into the pressurized fluid upstream of the nozzle at a flow rate in the range of 45 grams per minute to 180 grams per minute while the pressurized fluid is discharged through the nozzle.
Generally, the nozzle is oriented so that the pressurized fluid discharge from the nozzle impinges upon the target surface of the workpiece at an angle of at least 20 degrees with respect to the target surface. In addition, the nozzle is arranged with respect to the workpiece support so that the texturing standoff distance between the nozzle and the target surface of the workpiece is preferably at least 25 mm, and preferably no greater than 200 mm. This distance allows sufficient expansion of the periphery of the pressurized jet stream to avoid cutting or machining effects, while still allowing for sufficient deformation and hardening of the workpiece surface. In addition, at least one of the nozzle and the workpiece support can be moved during the discharge of the pressurized fluid so that the pressurized fluid moves across the target surface of the workpiece in a cross-hatch pattern as the pressurized fluid impinges upon the target surface. Therefore, the texturing (i.e., deforming) and hardening can be applied to the entire target surface.